article: Do you know why different animals or pests have their special colours? Colours in them seem to be mainly used to protect themselves.
  Some birds like eating locusts , but birds cannot easily catch them. Why? It is because locusts change their colurs with the changes of the colours of crops .When crops are green, locusts look green .But as the harvest time comes, locusts change into the same brown colour as crops have .Some other pests whose colours are different from plants are easily found and eaten by others .So they have to hide themselves for lives and appear only at night.
  If you study the animals' life, you'll find the main use of colours is to protect themselves .Bears, lions and other animals move quietly through forests .They cannot be easily seen by hunters because their colours are much like the trees.
  Colours are useful not only on the land , but also in the sea .A kind of fish in the sea can give out a kind of black liquid when the fish face danger. The liquid spreads over quickly, so they cannot be found by their enemies and can quickly swim away. That is why they can live safely though they are not strong at all.
question: From the passage, we can know that  _  .
options: locusts are big animals locusts are easily found by birds locusts are dangerous to their enemies locusts can change their colours to protect themselves